Mice of the inbred SJL/J strain have a high incidence of a proliferative disease affecting several cell types, including reticulum cells and plasma cells, which is frequently accompanied by gamma(1) and gamma(2) paraproteinemia. In only some instances can seriallytransplantable lines of neoplastic cells be obtained; these are reticulum cell sarcomas. Mice with transplanted reticulum cell e arcomas do not have paraproteinemia and may develop profound hypogammaglobulinemia. The disease may be viewed as an abnormal proliferation of reticulum cells which differentiate into plasma cells with consequent paraproteinemia; the subsequent emergence of transplantable reticulum cell sarcoma appears as an end stage in which this capacity to differentiate is lost.